The use of cellular communication systems has grown rapidly in the past few years. When a subscriber carries a portable cellular telephone, the subscriber is accessible to other telephone users as long as the subscriber is in the cellular communication network. The mobility of the cellular telephone is a distinct advantage over conventional landline based telephones, which require that the user remain at or near the place where the telephone is connected to the landline network. The cellular communication can offer this improved mobility because the communication medium is wireless.
FIG. 8 is an illustration of a conventional cellular communication system.
A typical cellular communication system 800 can be thought of as a hierarchical network. A subscriber typically carries a mobile unit 802b such as a cellular phone. In order to gain access to the cellular communication network 800, the mobile unit 802b provides the user access to the cellular communication system 800. The mobile unit 802b interfaces with a base transceiver station (BTS) 804b.
The BTS 804b provides coverage for multiple subscribers in a specific geographic area, called a cell. As the mobile unit 802b enters the cell, the BTS 804b and the mobile unit 802b communicate with one another. Information from this initial exchange is used by the cellular communication system 800 so it can route calls to and from the mobile unit 802b.
The BTS 804a has a limited coverage area. As a result, one technique for providing coverage for a large geographic area is to install multiple BTS units. This strategy also provides the benefit of increasing capacity, so that the cellular system 800 can serve a larger number of subscribers within its coverage area. However, a major drawback to this solution is the high cost of a BTS. As an alternative, a repeater 806 is often used to improve coverage area, reduce cost, and improve clarity. Generally, repeaters receive a downlink signal from a BTS and retransmit the downlink signal to a mobile unit after the downlink signal has been amplified by a downlink amplifier. The process works similarly in reverse, where the repeater will amplify an uplink signal from a mobile unit to the BTS with an uplink amplifier. The amplifiers provide an increase in signal strength which improves the clarity of the calls and prevents dropped calls. To distinguish the direction of the signals transmitted and received by the repeaters, the path between the repeater and the BTS is referred to as the "backhaul" signal path.
In order to control and coordinate the multiple BTS units exemplified by 804a, 804b, they are interfaced with a base station controller (BSC) 808. The BSC 808 controls the wire and radio links between the multiple BTS units 804a, 804b and a Mobile Switching Center (MSC) 810.
The MSC 810 performs call processing functions such as transcoding and soft-hand-off. The MSC 810 also has a location register, where it stores the location information to track the location of the subscriber's mobile unit 802 throughout the cellular communication network. The MSC 810 also provides an interface to an external network. The external network is typically a landline phone network such as the public switched telephone network (PSTN) or integrated services digital network (ISDN).
The MSC 810 is also often interfaced with an Operation and Maintenance Center ("OMC") 812. The OMC 812 allows network engineers to oversee and maintain the cellular communication system 800. The OMC 812 monitors such things as call traffic, status, and fault processing. If the cellular communication system 800 is not operating within normal parameters, the OMC 812 would investigate and attempt to correct.
In order to sustain the increased use of cellular communication systems, providers have to ensure that the cellular communication system is fully functional at all times since subscribers will not tolerate an unreliable system. Maintaining full functionality becomes more difficult as the cellular communication systems become physically larger and spatially more diverse to accommodate the increased capacity of new users.
An example of the difficulty in maintaining the functionality of the cellular communication system is the testing and monitoring of repeaters. Repeaters serve to extend the range of coverage that can be provided by a single BTS unit. Therefore, it is important to keep the repeater functioning in order to maintain communication with the users in the outer or fringe areas of a cell. However, repeaters are often used in cellular communication networks because of their lower cost compared to a BTS. As a consequence, repeaters are often not designed with remote testing and monitoring capability.
Cellular communication network providers have attempted to increase the remote testing and monitoring capability of repeaters by attaching modems and telephone lines to the repeaters. In addition to testing that is required at initial deployment, a repeater may have sub-system functions or modules which, absent remote testing and monitoring capability, can only be tested or monitored on-site. On-site testing of remotely-located repeaters typically requires that a technician travel to the repeater site and employ specialized test equipment. Further, on-site testing usually requires that the repeater being tested be removed from service during the testing. In order to achieve remote monitoring, a telephone line is connected to the repeater via a modem. In the event of an error or failure within the functions or modules, the repeater would issue an alarm or error message through the modem and over the telephone line to an OMC. The OMC receives the error message and dispatches a repair crew.
Alternatively, an OMC could use the telephone line connection with the repeater to place the repeater in a test mode and attempt to locate the problem with a repeater in a given cellular communication network. However, both uses require that each repeater have an attached telephone line and an installed modem which adds to both the initial installation costs and the monthly maintenance costs of the cellular communication network.
A clear example of the difficulty with the testing and monitoring of repeaters is the testing of the uplink and downlink RF paths. This testing must be done at initial deployment of the repeater, and is often necessary subsequently, to adapt the repeater to changes in the cellular communications system. The repeater's uplink and downlink RF paths are tested to ensure that the repeater is properly detecting and receiving the appropriate input range power and outputting the appropriate output range power. To perform this testing, low-level signals are injected at the antenna inputs and high-power signals are measured at the antenna outputs. This type of testing typically requires specialized RF test equipment and a visit to the actual remote repeater site which adds to the difficulty of testing because of the equipment and manpower that needs to be brought to the repeater site. In addition, the repeater is usually taken "offline" during the testing which disables a part of the cellular communication network.
Whereas wireless repeaters use RF backhaul signals to communicate with a BTS, landline repeaters are connected to a host BTS by a wireline connection. Landline repeaters have successfully employed a solution to the problem of on-site testing and monitoring. U.S. Pat. No. 5,422,929 ('929) to Hurst et al. describes a method and apparatus for remotely testing and monitoring a landline repeater. A central office will send an interrogating signal with an address subfield. When the interrogating signal is recognized by a controller in the landline repeater, the controller causes the landline repeater to enter a loopback mode where diagnostic and test-indicative no-operation signals are returned. For a given landline, any repeater attached to the landline could be addressed and tested. Although '929 describes the testing and monitoring for landline repeaters through the existing landline communication channel, '929 does not describe the testing and monitoring through a wireless communication channel and testing of the uplink and downlink paths of a wireless repeater.
U.S. Pat. No. 5,785,406 ('406) to DeJaco et al. describes a method and apparatus for testing through a wireless communication channel. In the '406 patent, a test signal is generated from a monitoring station located on a PSTN. The test signal is routed through the PSTN to a cellular communication system to a cellular phone. The test signal activates a loopback element within the cellular phone and the signal is re-routed back to the monitoring station. The monitoring station performs an analysis on the returned test signal.
Although the '406 patent describes the use of the loopback element in a mobile cellular phone through a wireless communication channel, '406 fails to disclose this loopback element for a wireless repeater. Furthermore, '406 fails to disclose how to implement this testing for a repeater and for testing the uplink and downlink paths of the wireless repeater.